1. Field of the Invention
The present invention relates to a method of producing metal slabs in which especially a metal strip of steel is conducted through a bottom entry device of a vessel which is filled with melted metal, particularly steel, and, after the molten steel has crystallized onto the metal slab, the coated metal slab, particularly the coated metal strip, is pulled off above the vessel, wherein the crystallization carrier, i.e., the core strip, is conducted though the bottom entry device of the vessel which provides a clear opening width between the core strip and the entry device. The present invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
The method and apparatus described above are used predominantly for coating strips, but also for coating sectional members and wire, preferably of steel. The strip, for example, of carbon steel, is conducted through the bottom of a vessel filled with molten steel having the same quality as the strip or a different steel quality, for example, stainless steel, and is for a certain period of time brought into contact with the molten steel whose temperature is controlled in order to coat the strip.
A method and apparatus of this type are the method and device for producing thin metal slabs in accordance with EP 0 311 602 B1. In this method, the bottom of the crystallizer (vessel with molten metal) is mechanically closed relative to the strip traveling therethrough. This mechanical contact can be achieved by means of a body of solid material, such as, a refractory stone, or also of steel, in a sliding or rolling manner.
DE 44 26 705 C1 discloses an inversion casting device with crystallizer. In this case, an uncooled purified metal strip having a low heat content is removed from a metal roll and is guided through molten metal contained in the crystallizer. When the metal strip makes contact with the molten metal, the molten metal crystallizes onto the relatively cool metal section. The thickness of the crystallization depends on the duration of the contact time as well as the temperatures of the metal section and of the molten metal. In this known inversion casting device, a seal horizontally surrounding the crystallizer is provided near the bottom thereof. Nozzles are directed from the seal toward the interior of the crystallizer. The openings of the nozzles are arranged in such a way that the molten metal flowing out of the nozzles impinge at an acute angle onto the carrier strip in the strip travel direction, so that with a relative speed of almost zero the molten metal can crystallize onto the strip. The bottom of the crystallizer is provided with an entry for the metal strip which with a mechanical seal prevents the molten metal from flowing out of the crystallizer.
DE 195 09 691 C1 discloses an inversion casting device with crystallizer having a bottom entry for the metal strip in the crystallizer formed by a slot-shaped duct, wherein there is little contact between the metal strip and the duct, and wherein the molten metal is cooled in the area of the opening of the duct to a temperature in which a two-phase area is present whose crystal component is between 50 and 90%, wherein the metal strip comes into contact in the area of the opening of the duct with this cool quantity of molten metal. The two-phase area should have such a high viscosity that it assumes the function of a seal which renews itself and prevents penetration of the molten steel into the gap and the bottom entry.
DE 195 09 681 C1 discloses another inversion casting device with a crystallizer which is filled with molten metal and in which the carrier strip is preheated to a temperature of about 200.degree. C. before the strip is introduced into the bath of molten metal. Preheating of the carrier strip takes place by means of an indirect heat exchange in the oxygen-free surrounding. For this purpose, the carrier strip is conducted through a relatively long duct arranged perpendicularly in the crystallizer. In the vicinity of the entry point of the carrier strip from the heat transfer duct into the molten metal, a meniscus is formed which is in the two-phase area of the molten metal with an isothermal line which is between the liquidus temperature and the solidus temperature. As is the case in DE 195 09 691 C1, this two-phase area has such a high viscosity that it is supposed to assume the function of a seal which renews itself in order to prevent the molten metal from flowing out of the crystallizer.
Each of the above-described examples of solving the problem of preventing molten metal from flowing out of a crystallizer of an inversion casting device has specific disadvantages.
Thus, in the case of the mechanical seal, it is difficult to realize a uniform movement of the strip to be coated and the wear at the friction-type seal is too high.
On the other hand, in the case of the partial undercooling of the molten metal in the vicinity of the bottom entry for the strip to be coated into the crystallizer, the temperature control is very difficult to carry out, particularly when the temperature difference between liquidus temperature and solidus temperature is a relatively small two-phase area, as it occurs especially in low carbon molten steels (0.005-0.2% C.). In addition, there may be the danger of presolidification and regulus formation at the crystallization carrier.